PTM transmissions such as multicast or broadcast transmissions are used for distributing data from typically a single source to a plurality of receivers. In a typical scenario, some content in the form of multimedia data is distributed to an audience within the framework of a (mobile) TV service, Newsfeed, Podcast service, etc. The users may or may not have to register for the service. The content data may be provided in the form of one or more media files representing audio data, video data, image data, but may also contain other data such as any type of binary data such as compressed data files, software files including some piece of software for execution on a processing host, etc. On the other hand, also a simple text file which is distributed by, e.g., a Newsfeed service adapted to bandwidth-limited conditions in a mobile network may be regarded as content.
The content data distributed via a PTM transmission needs to be free from transmission errors to a very high degree. For example, in resource-limited transmission environments such as mobile networks large content files such as image or video files are compressed before transmission using a suitable compression algorithm, e.g. MPEG-2, MPEG-4, MP3, GZIP, etc. The corresponding decompression in the receiving device will in general not be able to handle with transmission errors due to the complex interrelations of multiple data sets (e.g., neighbouring frames in a video) induced by the compression algorithm. Even a single 1-bit error may result in the situation that the received file cannot be opened for presentation to the user. Un-compressed files are also vulnerable to transmission errors; consider for example a multimedia file whose header information is corrupted: Also this file will most probably not open.
In order to reduce transmission errors in data transmission environments, some form of error control is required. Error control enables a data receiver to perform error detection and possibly also error correction in case of transmission errors. Techniques for error control are of general importance in many data transmission environment. Some environments are more prone to errors than others; for example a wireline transmission will generally produce less errors per given amount of data than a wireless transmission. Error control schemes are available which have proven particularly useful in the one or the other environment in order to achieve the desired degree of error-free transmission.
As an example for mobile network environments, the UMTS (Universal Mobile Telecommunications Service) MBMS (Multimedia Broadcast/Multicast Service) in 3GPP Release 6 requires that a conforming mobile network implements two mechanisms for increasing the transmission reliability: the first mechanism comprises to add FEC (Forward Error Control) data to the transmitted MBMS content data; the second mechanism is a post-delivery file repair procedure.
Regarding the first mechanism, the FEC data represent redundant information which is provided together with the content data in order to enable the receiver to reconstruct the content data in case of an transmission error. For example, 5 content data bits may be transmitted together with 3 redundancy bits, wherein the redundancy bits (the redundancy data) has been calculated from the 5 content data bits according to a predetermined FEC algorithm. The receiver applies a corresponding algorithm to detect errors in the 5 data bits and to possibly correct the error or request re-transmission, depending on the specific procedure.
Within the framework of the MBMS file repair procedure, a receiving device detects a transmission error, for example because a received filed cannot be decompressed or the header of the file appears to be corrupted (or some bit errors are detected because the content data comprises some redundancy data for detection, but not correction, of transmission errors). The receiving device may then request a re-transmission of the corresponding file. This includes that the device sends a file repair request to the originator of the content data controlling the PTM content transmission in the network. This entity is called BM-SC (Broadcast/Multicast Service Centre) in the MBMS field. The BM-SC may use a PTP (point-to-point) connection for re-transmission of the requested file to the receiving device. In case too many receiving devices request a re-transmission of the file, the BM-SC may also decide to use an MBMS bearer for re-transmission, i.e. performs another broadcast or multicast content data transmission.
Adding redundancy data (FEC data) to the content data is problematic, as it increases the transmission resources required over the radio interface to a considerable degree. Still then, the amount of redundancy provided may be insufficient for successful error correction in case a receiver is in a location with difficult transmission conditions. On the other hand, the added redundancy data may be superfluous for receiving devices in more favourable transmission conditions, in which case the added redundancy data wastes transmission resources.
Requesting a re-transmission of data blocks or even entire files of content data requires the setup of typically many extra PTP connections or even another PTM transmission. This leads to a considerable amount of signaling in the network, usage of processing resource in the control node of the content data transmission (e.g., BM-SC) and further involved nodes in the network. Further, a re-transmission poses a considerable delay in error recovery, which is made worse in case of the MBMS file repair procedure by a mandatory back-off time after the end of the PTM content data transmission in order to avoid network congestion from an implosion of too many parallel file repair requests.